Yes. I do use the BOOX driver (BeBook Neo is same hardware as Boox 60 with "customized" firmware).

It seems that the value in the "comma separated list" is used only when reading/writing to the main memory, but doesn't work when the SD card is involved.

calibre gets the list of books on device scanning only the dir MyBooks on main (as I like), but scanning every directory on the card, no matter where it is.

Note that it's the same in writing, the value in the "comma" list is used writing to main memory, but to put my book on a directory the dir name must be in the template (in the row above the "comma" one), like you suggested in the previous post:
MyBooks/{author_sort}/{title} - {authors}
(the same if I put the template in the save parameter in preferences).

This is what happens on my BeBook Neo (fw version 1.4). I don't know if it works the same on Onyx, but I can guess so, since the Neo fw should be very similar to the Onyx one (just a rebranding and different links to web sites). May be someone can do the test on the Boox.

Any further suggestion?

ps: is there any way to use custom columns values in the save to device template?
something like:
MyBooks/{#myaddedcolumn}/{title} - {authors}